A vibrational spectrum of molecules, ionic compounds, and polyatomic ions may be generated using Raman spectroscopy. This vibrational spectrum may be used to identify a sample of a unique chemical or a unique mixture of chemicals much as a fingerprint is used to identify a person.
To generate a Raman spectrum, a sample is irradiated with a monochromatic excitation source. This monochromatic excitation source provides spectral resolution in the Raman spectrum. By contrast, a broadband source may generate broad vibrational peaks in the vibrational spectrum.
Typically, when photons from a monochromatic excitation source impinge a sample, a majority of the photons are scattered elastically resulting in a Rayleigh scattering of light. This scattering of light has the same wavelength as the excitation source. The excitation source therefore should be relatively intense in order to generate a Raman signal that may be relatively easily detected. As a result, lasers are almost exclusively used as the monochromatic excitation source in conventional Raman spectrometers.
The efficiency of Raman scattering is inversely proportional to the laser wavelength raised to the fourth power. It is easier therefore to generate a detectable signal using a laser with a relatively short wavelength. A short wavelength laser, however, may have a relatively large amount of energy per photon, which can result in the generation of fluorescence due to the population of excited electronic states within the sample or within impurities included in the sample. Fluorescence generation is typically many orders of magnitude more efficient than Raman scattering. Using a short wavelength laser therefore often results in a spectrum with a fluorescence signal that is much larger than the Raman signal and may prevent the Raman signal from being accurately measured. Thus, there are both advantages and disadvantages for any laser wavelength that might be selected. Many commercial instruments therefore include a plurality of excitation lasers so that a user may select which laser wavelength is appropriate for a particular measurement.
The Raman signal is detected with a detector sensitive to the Raman photons generated by the laser. A typical detector includes a plurality of charge-couple-devices (CCDs). These CCDs enable a spectrum with a relatively high signal to noise ratio to be obtained due to the nature of the CCDs. For example, a silicon-based CCD detector may be made in an array format so that Raman photons of different wavelengths may be detected substantially simultaneously by dispersing the photons spatially across the CCD. This multiplex effect allows the entire Raman vibrational spectrum of a sample to be detected with a single measurement.
For a sample that gives rise to strong fluorescence as discussed above, it is desirable to use a long wavelength laser so that the excited states of the sample are not populated efficiently. A typically CCD detector, however, is not sufficiently sensitive to photons with a wavelength greater than 1100 nanometers (nm). This presents a dilemma since it is common to distinguish chemicals based on vibrations due to carbon-hydrogen (CH), oxygen-hydrogen (OH), and nitrogen-hydrogen (NH) bond stretching. These vibrations generally occur at absolute energies between 2700-3300 wave numbers. When used in combination with vibrations of lower energy (e.g., the “fingerprint” region), a significant advantage is realized.
To observe the hydrogen stretching vibrations with a CCD detector that detects photons at wavelengths shorter than 1100 nm, the excitation laser should be less than 807 nm. For this reason, one of the most common lasers used in an excitation source is a 785 nm wavelength laser. Use of a 785 nm wavelength laser, however, often results in generation of significant fluorescence. Thus, it is desirable to use a longer wavelength laser in order to reduce fluorescence. Use of a longer wavelength laser, however, may result in the loss of important vibrational information about the chemical sample since the vibrations due to carbon-hydrogen (CH), oxygen-hydrogen (OH), and nitrogen-hydrogen (NH) bond stretching occur at wavelengths which are not detectable by a silicon CCD. Furthermore, less expensive CCDs typically cannot efficiently detect photons with wavelengths above 1060 nm. For example, the CCDs typically used in handheld Raman instruments cannot detect the laser wavelengths greater than 785 nm that include the important CH, OH, NH bond stretching vibrations.
Longer wavelength lasers in combination with detectors that are sensitive to longer wavelength photons may be used during Raman spectrometry. For example, an FT-Raman spectrometer typically includes a 1064 nm wavelength laser and a single element detector based on germanium or indium gallium arsenide in combination with an interferometer. Although FT-Raman spectrometer detects substantially the entire Raman spectrum including the CH, OH, and NH stretching region, this spectrometer has other deficiencies. In particular, the laser wavelength of the FT-Raman spectrometer is so long that in order to generate a sufficient Raman signal, a high optical power is used that may result in sample burning. This problem is compounded since germanium (Ge) detectors and indium gallium arsenide (InGaAs) detectors generate a higher noise level than silicon detectors such as CCDs. This poor performance relative to a CCD detector results in a lower quality spectrum or the requirement of a much longer acquisition time.
In a second example, a spectrometer is configured with InGaAs array detectors and a long (e.g., 1064 nm) wavelength laser. Although the InGaAs array detectors provide a multiplex advantage similar to a CCD, this spectrometer may also cause sample burning as described above. Furthermore, even with the addition of the multiplex effect, the InGaAs array detectors are still noisier than CCD detectors and thus have a worse quality signal-to-noise ratio. This is compounded by fixed pattern noise due to non-uniformity of the individual InGaAs detector elements (pixels) which make the InGaAs array detector.
In a third example, a spectrometer is configured with a short wavelength laser (e.g., 785 nm wavelength laser) with a CCD detector, and a long wavelength laser (e.g., 1064 nm wavelength laser) with an InGaAs array detector. This configuration allows a user to select traditional CCD detection (e.g., with 785 nm laser excitation) for samples which exhibit negligible or low fluorescence, or to select InGaAs array detection (e.g., with 1064 nm laser excitation) for samples which exhibit significant fluorescence. Effectively providing two Raman spectrometers, however, dramatically increases cost, size, and complexity of the spectrometer. In addition, this spectrometer still suffers from the poor signal to noise of the InGaAs array detector for part of the vibrational information.
As described above, prior art spectrometers are unable to use laser wavelengths long enough to significantly reduce fluorescence relative to a 785 nm wavelength laser, while still being detectable by a CCD detector and providing the vibrational spectral region including CH, OH, and NH vibrations. In addition, these spectrometers also suffer from a spectral resolution problem. For example, since the Raman signal is dispersed spatially across the CCD as a function of wavelength, the resolution between vibrational peaks may be limited by the size of the CCD detector. Therefore, in order to observe both the fingerprint region and the hydrogen stretching region with good resolution, a relatively large CCD detector should be used (e.g., one inch or larger in length). However, such a relatively large CCD adds significant expense to the spectrometer.
There is a need in the art for an improved method and apparatus for acquiring Raman spectra.